The disclosure of Japanese Patent Application No. 2002-043216 filed on Feb. 20, 2002 including the specification, drawings and abstract is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to a charge/discharge control apparatus and method for a battery pack in which a plurality of battery cells are connected in series, and to a control method thereof.
2. Description of the Related Art
Hybrid vehicles incorporating an electric power generator driven by an engine, in addition to a vehicle-driving electric motor, have been known. Such a hybrid vehicle has a battery pack in which a plurality of battery cells are connected in series. Using electric power from the battery pack, the vehicle-driving electric motor is driven. Electric power from the generator is stored into the battery pack.
FIG. 3 illustrates the construction of a charge/discharge control apparatus for a battery pack as mentioned above. Referring to FIG. 3, a battery pack 12 is formed by a plurality of battery cells 10 connected in series. A voltage detector 14 for detecting the voltage of the individual cells 10 is connected to the battery pack 12. On the basis of an output of the voltage detector 14, a battery ECU 16 computes the state of charge SOC of the cells 10.
Normally, the battery pack 12 has an arrangement in which a plurality of battery blocks each formed by a plurality of battery cells 10 connected in series are connected in series. Therefore, the value of voltage detected by the voltage detector 14 may be the value of voltage of each cell 10 as mentioned above, or may be the value of voltage of each battery block. In the latter case, the battery ECU 16 computes the state of charge SOC of each battery block from the value of voltage of the battery block. On the basis of the thus-computed SOC value, an HVECU 18 controls loads 20 that include a vehicle-driving electric motor, a generator-driving engine, a generator, an inverter, etc., and performs control of adjusting the amount of charge of the battery pack 12 so that the SOC value of the battery pack 12 is within a predetermined range. A typically adopted control range of SOC is, for example, the range of 20% to 80%. An SOC control method for the battery pack 12 as described above is disclosed in Japanese Patent Application Laid-Open Publication No. 2000-14029.
The foregoing related-art control apparatus performs control such that the SOC of the battery pack 12 is within a predetermined range (e.g., of 20% to 80%). In this case, a battery block that exhibits the lower limit value (20%) of the control range is different from a battery block that exhibits the upper limit value (80%) of the control range. That is, if a control is performed such that the SOC of the battery pack 12 will not fall below the lower limit value of the control range, the control is performed as follows. That is, the remaining capacity of each battery block is determined from the voltage thereof. The SOC value of the battery block that exhibits the least value of the thus-determined remaining capacities is set as a least SOC value for use in the control of the SOC of the battery pack 12. The control is performed so as to prevent the least SOC value from falling below the lower limit value. If a control is performed such that the SOC of the battery pack 12 will not exceed the upper limit value of the control range, the control is performed as follows. That is, the SOC value of the battery block that exhibits the greatest value of the remaining capacities of the battery blocks is set as a greatest SOC value of the battery pack 12. The control is performed so as to prevent the greatest SOC value from increasing above the upper limit value.
The charge/discharge capabilities of the individual cells 10 vary with increasing time of use of the battery pack 12. That is, the variation in SOC among the cells 10 or the battery blocks is small as indicated in FIG. 4(2) during an initial period of use of the battery pack 12. However, as time elapses, the variation in SOC becomes greater as indicated in FIG. 4b. 
In the case of the above-described related-art charge/discharge control apparatus, however, it is necessary to compute both the least value and the greatest value of SOC of the cells 10 or the battery blocks. Moreover, it is necessary to perform the control regarding the lower limit value of SOC through the use of the least value and the control regarding the upper limit value of SOC through the use of the greatest value separately from each other. Therefore, the related-art apparatus has a problem of complicated computation processes.
Furthermore, the variation in SOC among the cells 10 or among the battery blocks becomes great as indicated in FIG. 4b. Correspondingly, the SOC control precision decreases. Therefore, it becomes necessary to perform a uniformalization control of reducing the SOC variation among the battery cells by overcharging the cells 10 or the battery blocks. Due to the overcharging of the cells, the uniformalization control gives rise to a problem of reduced service life of the cells.
Accordingly, it is an object of the invention to provide a charge/discharge control apparatus for a battery pack which is capable of performing SOC control and reducing the SOC variation through simple processes, and a control method thereof
In order to achieve the aforementioned object, a charge/discharge control apparatus for a battery pack having a plurality of cells according to one aspect of the invention includes a voltage detection portion that measures a voltage of each set of a predetermined number of cells provided in the battery pack, a computation portion that computes a state quantity of each set of the predetermined number of cells based on the voltage measured by the voltage detection portion, and a charge/discharge control portion that performs a charge/discharge control based on a least state quantity that is a least one of the state quantities of the sets of the predetermined number of cells.
According to the above-described construction, the charge/discharge control of the battery pack is performed using only the least one of the state quantities of the cells or the battery blocks of the battery pack. Therefore, the computation of SOC can be simplified. Furthermore, although execution of control of the upper limit value of SOC through the use of the least state quantity tends to further increase the amounts of charge of the cell with the greatest capacity value and cells having capacity values close to the greatest capacity value, actual SOC increases are curbed due to increases in self-discharge and decreases in charging efficiency. Therefore, the apparatus of the invention is able to curb the SOC variation without performing the uniformalization control by overcharging the cells.
In the above-described charge/discharge control apparatus for the battery pack, each set of the predetermined number of cells may be a single cell or a battery block in which a plurality of cells are connected in series.
Furthermore, in the charge/discharge control apparatus, the state quantity may be a remaining capacity value of each set of the predetermined number of cells based on the voltage, or may be a percentage of the remaining capacity value to a fully charged capacity value.
Further, the charge/discharge control apparatus may further include an overcharge determining portion that performs an overcharge determination as to whether an overcharged state exists based on a greatest state quantity that is a greatest one of the state quantities of the sets of the predetermined number of cells.
According to this construction, the overcharge determination is performed on the basis of the greatest one of the state quantities of the sets of the predetermined number of cells. Therefore, it becomes possible to prevent the service life reduction of the cells caused by the overcharging of the cells of the battery pack.
In accordance with another aspect of the invention, a charge/discharge control method for a battery pack having a plurality of cells includes measuring a voltage of each set of a predetermined number of cells provided in the battery pack, computing a state quantity of each set of the predetermined number of cells based on the voltage measured, and performing a charge/discharge control based on a least state quantity that is a least one of the state quantities of the sets of the predetermined number of cells.
In the above-described charge/discharge control method for the battery pack, each set of the predetermined number of cells may be a single cell or a battery block in which a plurality of cells are connected in series.
Furthermore, in the charge/discharge control method, the state quantity may be a remaining capacity value of each set of the predetermined number of cells based on the voltage, or may be a percentage of the remaining capacity value to a fully charged capacity value.